The recovery of oil and gas from underground geological formations is of great importance in modern society, which uses vast amounts of fossil fuels for its essential energy. The individual well productivity declines over a period of time because of a number of factors, including changes in reservoir fluid characteristics, depletion of reservoir fluid characteristics, depletion of reservoir energy, decreasing premeability of the formation to the oil, the gradual dissipation of the expanding pressure transient, contamination of the well bore, reduced permeability of the oil through the region immediately surrounding the well bore and reduction of the internal diameter of the well pipe.
The response to the declining productivity was the development of numerous techniques, which has become collectively known as well workover and stimulation. The concept of fracturing or formation breakdown has been recognized to play a very important role in the application of these oil production enhancement techniques, including stimulation, acidizing, water injection and cementing of the formation.
Hydraulic fracturing has found wide usage as a well stimulation procedure for creating deep-penetrating fractures (both horizontal and vertical) that provide high capacity channels for flow from deep within the producing formation to the well as for overcoming damaged matrix permeability surrounding a wellbore. In order to produce gas or liquids from a well at a higher rate following a hydraulic fracturing treatment, the reservoir must contain enough fluids in place and the formation must not have regions of severe permeability reduction, particularly in regions adjacent to the well. Early experimental work in shallow wells demonstrated that a hydraulically formed fracture tends to heal --that is, to lose its fluid carrying capacity after the parting pressure is released -- unless the fracture is propped. Typical propping agents for retaining the integrity of the fractures are nutshells, plastic beads, aluminum spacers, glass beads, sand and urea prills.
Proppants thus provide a means for meeting the objective of the fracturing which is to increase the well production by preventing collapse of the formation and resultant decrease in fluid permeability.
It is common practice to fracture oil wells using a water-based brine fluid containing a water-soluble polymer such as hydroxypropyl guar cross-linked with a transition metal salt as is wellknown in the art.
Traditionally, the introduction of reagents downhole during fracturing and other well stimulation processes is carried out by forcing a solution of the reagent down the hole and into the formation, whereupon it becomes absorbed onto the formation and released slowly from it. Unfortunately, the rate of release is variable and generally is quite fast.
It is known to force plastic beads of ethylene-vinyl acetate copolymers into the formation, but these beads are generally too large to get into the fractures formed in the formation since smaller beads would dissolve too rapidly. Further, the beads are too soft for the proppant function.
When fracturing is complete and the well is put into production, the flowing connate water may contain unstable salts which can precipitate and form scales on the production tubing. It is possible to inhibit the formation of scale by introducing a phosphonate scale inhibitor continuously downhole at a level of a few parts per million. It would be preferred to place the scale inhibitor downhole in a solid form during the fracturing operation and allowing it to leach back with the produced water; however, the scale inhibitor is too quickly solubilized to have a lasting effect in the downhole application.
It is known from British Patent Specification No. 1,290,554 to inhibit scale formation downhole by supplying downhole a solid linear carboxylic polymer having low molecular weight and in which the carboxylic groups are neutralized by an alkaline earth or other insolubilizing cation to an extent such that the polymer has a controlled low solubility in water. It is stated in that specification that water-soluble scale inhibitors may also be supplied downhole with the substantially water insoluble polymer beads.
Beads containing an oil-soluble reagent having wax crystal modifier activity dispersed in a polymeric matrix have been sold in the United States of America and used therein for oil production from an underground geological formation to inhibit the deposition of wax from the oil onto the walls of structures through which the produced oil was passed.
It is an object of this invention to provide an article and its use to enhance the production of hydrocarbons from geological reservoirs, more particularly from fractured formations.
It has been an additional object to devise a composition for providing controlled release of a reagent downhole, in a pipeline, in other fluids containing condensed water.